Fuel cells are well known and are commonly used to produce electrical current from hydrogen containing reducing fluid, fuel and oxygen containing oxidant reactant streams, to power various types of electrical apparatus. Many fuel cells use a liquid electrolyte such as phosphoric acid, and such fuel cells are typically adjacent other fuel cells to form a well known fuel cell stack having manifolds and associated plumbing to deliver and remove reactant and exhaust stream, etc.
Phosphoric acid electrolyte fuel cells are frequently associated with a problem of migration of acid out of one cell into an adjacent cell. Many efforts have been undertaken to resolve this problem. Such efforts are disclosed, for example, in commonly owned U.S. Pat. No. 5,079,104 to Roche et al., U.S. Pat. No. 5,156,929, to Dec et al., U.S. Pat. No. 5,178,968 to Roche, U.S. Pat. No. 5,270,132 to Breault et al., U.S. Pat. No. 5,837,395 to Breault et al., and U.S. Pat. No. 6,050,331 to Breault et al., which patents are hereby incorporated herein by reference thereto.
Such phosphoric acid fuel cells contain excess acid to accommodate acid loss due to evaporation into the reactant streams, loss due to absorption by cell components and loss by reaction with materials within the cell. This excess acid is stored in electrolyte reservoir plates which may be a separate component or the electrolyte storage function may be integrated into the porous electrode substrate. Managing the liquid electrolyte within a fuel cell is a significant design challenge.
The carbonaceous materials at the edges of planar components of the fuel cell that are exposed to the air inlet and air exit manifolds are oxidized due to chemical reaction. The extent of oxidation is a function of the electrochemical potential, the partial pressure of water vapor and the local temperature. Oxidation is normally greater at the air inlet edge of the cell due to higher temperatures than at the air exit edge of the cell. Oxidation typically is minimal on the edges exposed to the reactant fuel.
Oxidation of the carbonaceous material results in the material at the edge of the cell becoming wettable and leads to the presence of an acid film along the edge of the separator plate assembly. This acid film results in an ionic shunt current path along the edge of the fuel cell. This shunt current path results in protons (hydrogen ions) flowing from the positive end of a substack of cells to the negative end of the substack along the edges of the cells. A “substack” of fuel cells is a group of typically 4-8 cells disposed between cooling plates within the fuel cell stack. There are two consequences to these shunt currents. The first consequence is that the shunt current lowers the electrolyte potential such that carbon corrosion occurs at the positive end of the substack. Carbon corrosion is a significant issue for fuel cells operating at elevated reactant pressures where the electrode potentials are higher than at ambient pressure. The second consequence is that the shunt current results in the flow of anions (di-hydrogen phosphate) from the negative end of the sub-stack to the positive end of the sub-stack. The hydrogen ions and the di-hydrogen phosphate ions combine at the positive end of the sub-stack. This results in acid being pumped from the negative end of the sub-stack to the positive end of the sub-stack along the edge of the stack. The consequences of this acid pumping is that the cells at the negative end will fail due to reactant cross-over due to the loss of acid; and the positive cell will fail due to poor performance due to the excess acid. Acid pumping from cell to cell significantly reduces the useful life of the fuel cell. The acid pumping problem is most severe in cells with small electrolyte reservoirs.
FIG. 1 presents a schematic representation of the above described acid pumping or acid migration between “Cell 1” and “Cell 2”, wherein such cells would be two of many fuel cells in a fuel cell stack assembly 10. It is noted that an “integral separator plate”, also referred to herein as a “separator plate assembly” 18 is located between the cathode electrode 12 of “Cell 1” and the anode electrode 14 of “cell 2”. While the separator plate assembly defines reactant passage flow fields for both adjacent electrodes separated by an impermeable layer, FIG. 1 shows schematically how acid migrates as a film along an edge of the integrated separator plate between “Cell 1” and “Cell 2”.